This is the 2007 version. Click here for the 2017 chapter 02 table of contents.

The Split-Brain Operation

The corpus callosum is the main fiber bundle connecting the two hemispheres of the brain, allowing the two halves to communicate. In the following picture, you can see a mid-saggital section (a view of a brain which has been sliced between the two hemispheres). Note that the wrinkles or convolutions extend down from the top of the brain until they reach the whitish area in the middle, the corpus callosum. That is the only place where the two cerebral hemispheres are connected.

What is the corpus callosum?

When the corpus callosum is cut, the hemispheres can no longer communicate directly. This is the "split-brain" operation.

What was the split-brain operation and what were the results of it?

The split-brain work was pioneered by Roger Sperry, who perfected the operation in animals. In the 1960s it was used on humans. Doctors decided to disconnect the two hemispheres of a 48-year-old war veteran in hope of relieving his uncontrollable seizures. The results were so spectacular and important for brain science that Sperry ultimately received the Nobel Prize, 20 years after the first split-brain operation.

Some patients acted as if they had two different personalities in the two hemispheres after the operation. Gazzaniga (1970) wrote:

Why did Gazzaniga "discretely" leave his patient's backyard?

Case 1...would sometimes find himself pulling his pants down with one hand and pulling them up with the other. Once, he grabbed his wife with his left hand and shook her violently, with the right hand trying to come to his wife's aid in bringing the left belligerent hand under control. Once, while I was playing horseshoes with the patient in his backyard, he happened to pick up an ax with his left hand. Because it was entirely likely that the more aggressive right hemisphere might be in control, I discretely left the scene-not wanting to be the victim for the test case of which half-brain does society punish or execute. (p.107)

Reuter-Lorenz and Miller (1998) noted that a split-brain patient has "two minds in the same cranium" and may experience conflicts such as one hand placing items in a grocery cart while another returns them to the shelf. However, these conflicts decrease with time after the operation. "In particular, it has been shown that one hemisphere is able to inhibit, or gate, the responses of the other." Therefore, the more competent hemisphere tends to take control of performance on a given task.

Paul Pietsch has a nice page on the split-brain research at:

Many other web sites about the split-brain research are less scholarly. Most seem to accept uncritically the "nonsense" about hemispheric specialization discussed earlier.

Under what circumstances was the split-brain operation performed?

Split-brain patients were always tested intensively before the surgery, and the operation was performed "only if all other opportunities for permanently abolishing the seizures appeared closed" (Gazzaniga, 1970). Why would a split-brain operation prevent seizures? In these patients, the epileptic activity bounced back and forth between the two hemispheres, across the corpus callosum. Cutting the corpus callosum prevented the activity from building up, greatly reducing the number and severity of seizures.

Although the operation worked in the sense that it abolished the seizures, it was obviously a radical and irreversible procedure. Within years after the initial split brain operations on humans, drug therapies were found to provide an alternative for most patients, and the split brain operation became more rare.

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Copyright © 2007-2011 Russ Dewey